Light-duty electrical contacts of silver and ruthenium oxide

ABSTRACT

A light-duty electrical contact material which consists of a mixture of silver and ruthenium oxide, the ruthenium oxide contact being in the range 0.1 to 13.0 atomic per cent. For reasons of economy, the preferred ruthenium oxide content is 1.3%. The material is produced by powder metallurgical techniques from fine, irregular silver powder and preferably ultra fine ruthenium metal powder. Fine ruthenium oxide powder can be utilized in place of the ruthenium metal powder.

United States Patent 11 1 Davies Dec. 11, 1973 [5 LIGHT-DUTY ELECTRICALCONTACTS OF 3,477,845 H/l969 Comey et al 75/173 A SILVER AND RUTHENIUMOXIDE [75] Inventor: Terrence Ardern Davies, Horton, PrimaryExaminer-15. Dewayne Rutledge England Assistant ExaminerE. L. Weise [73]Assignee: Square D Company, Park Ridge, Ill. AtmmeywHawld Rathbun [22].Filed: Oct. 21, 1971 21 Appl. No.: 191,564 [57] ABSTRACT A light-dutyelectrical contact material which consists [30] Foreign ApplicationPriority Data of a mixture of silver and ruthenium oxide, the ruthe- O t21 1970 G tB 49 660/70 nium oxide contact being in the range 0.1 to 13.0tea n am atomic per cent. For reasons of economy, the preferredruthenium oxide content is 1.3%. The material is produced by powdermetallurgical techniques from I fine, irregular Silver powder andpreferably ultra fina [58] Fleld of Search 75/173, 200/166 C rutheniummetal powder Fine ruthenium oxide pow [56] References Cited der can beutilized in place of the ruthenium metal powder.

2 Claims, No Drawings LIGHT-DUTY ELECTRICAL CONTACTS OF SILVER ANDRUTHENIUM OXIDE The invention relates to light-duty electrical contactmaterials and to methods of producing light-duty electrical contacts.

The invention provides a light-duty electrical contact material whichconsists of a mixture of silver and ruthenium oxide, the ruthenium oxidecontent being in the range 0.1 to 13.0 atomic per cent. For reasons ofeconomy, the preferred ruthenium oxide content is 1.3%.

The invention also provides a method of producing a light-dutyelectrical contact including the steps of mixing fine, irregular silverpowder with ultra fine ruthenium metal powder to provide a mixturehaving a fine, evenly dispersed ruthenium content in the range 0.1 to10.0 atomic per cent; compacting the mixture into a desired shape;heating the compacted shape in a suitable atmosphere for a period oftime to effect sintering of same; and internally oxidizing the sinteredcompacts to convert the ruthenium metal into ruthenium oxide.

The invention further provides a light-duty electrical contact which isproduced by the method outlined in the preceding paragraph.

The foregoing and other features according to the invention will bebetter understood from the following description of specific embodimentsof the invention.

The electrical contact material according to the invention which, aspreviously stated, is suitable for lightduty applications, consists of amixture of silver and ruthenium oxide, and the concentration ofruthenium oxide can vary from 0.1 to 13.0 atomic per cent. The contactmaterial is best fabricated by powder metallurgical techniques and thepreferred and most economical material is a material having a rutheniumoxide content of 1.3%. Vacuum and gas melting techniques are unsuitablebecause it is not possible to disperse the ruthenium phase finely andevenly throughout the silver.

Thus in a method according to the invention fine, irregular silverpowder, and ultra fine ruthenium powder are initimately mixed togethersuch that the ruthenium content of the mixture is in the range 0.1 to10.0 atomic per cent. The intimate mixing can be effected by dry tumblemilling for a period of time of the order of 2 to 24 hours.Alternatively, the intimate mixing of the powder particles can beeffected by dry tumbling in the presence of glass spheres, or by millingunder acetone.

The size and shape of the metal powder particles is of prime importancein the manufacture of optimum silver-ruthenium oxide materials and bothpowders should preferably be as fine as is economically possible. Thisin practice involves the use of precipitated silver of less than 300 or350 mesh (preferably less than 20 microns average intercept), andruthenium powder in the sub-sieve size range (preferably less than 2microns average intercept) with preferably no ruthenium powder particlesof a size greater than 5 microns diameter. The use of fine powdersensures that a fine, even dispersion of the ruthenium is obtained in thefinished contact material and facilitates the rapid oxidation ofsilver-ruthenium alloys which are to be internally oxidized to obtain afine, even dispersion of ruthenium oxide in the silver.

The powder mixture is then compacted, using molds, into the desiredshape for the electrical contacts. The compacting can for example beeffected at a pressure of the order of 10 to 20 tons per square inch togive green densities of the order of of the theoretical maximum density.

The contact compacts are then sintered by being heated in a neutral orreducing atmosphere, for example 90%N /10%H for a period of time of notless than one hour. The upper temperature limit for the sinteringoperation is 960.5C i.e. the melting point of silver. In order tomaximize the sintering rate a temperature just below the melting pointtemperature should be utilized, for example a temperature of the orderof 930C. The sintering process increases the density of the contactmaterial to between and of the theoretical maximum density, the actualdensity attained being dependent upon the sintering time andtemperature.

With silver-ruthenium alloys which have been sintered in a reducingatmosphere it should be noted that an increase in the dimensions of thecontact compact occurs during the subsequent internal oxidation step dueto an increase in the volume of the material when the ruthenium isconverted to ruthenium oxide, and to the generation of pockets of steamdue to the reaction of oxygen with any dissolved/residual hydrogen. Thelatter effect can be avoided by sintering the initial alloy in a neutralatmosphere.

Cold welding of the soft silver powder particles readily occurs whencompacting is effected at high pressures, for example 40 tons per squareinch, and this welding results in pockets of trapped air in the greencontact compact. The pockets of air are expanded during the sinteringstep and can, therefore, cause distortion and even expansion of thecompact. Thus it is important to ensure that high forming pressures ofthis order are avoided during the method according to the invention.

The sintered contact compacts are then internally oxidized by beingheated in air at a temperature of the order of 930C for a period of timeof not less than one hour. This oxidation process completely convertsthe sub-surface particles of ruthenium metal in the silver (Ag) toruthenium oxide (RuO- Metal particles at greater depths will only bepartially oxidised on their surfaces.

Ruthenium oxide is a conducting oxide which exhibits very low electricalresistivity and is contained in the contact material as a fine, evendispersion.

The density of the contact material may then, if dc sired, be increasedto at least of the theoretical maximum density by a stamping operationat a pressure of the order of 40 to 45 tons per square inch.

The material of the electrical contacts produced by this method exhibitslow, stable contact resistance at low contact forces over a period ofyears under atmospheric conditions which would normally tarnish andcorrode known silver base contact materials such as silver-cadmium oxideor silver alone.

In an alternative method according to the invention, the greensilver-ruthenium compacts can be compacted at a pressure of the order of10 tons per square inch, and then sintered in air for a period of theorder of one hour at a temperature of the order of 930C. This sinteringprocess simultaneously sinters the contact material and oxidizes theruthenium to RuO The ruthenium powder particles situated well below thesurface of the compacts are oxidized and the density of the contactmaterial is increased from 70% to 80% of the theoretical maximumdensity. I

As with the previous method, the density of the contact material maythen, if desired, be increased to at least 95% of the theoreticalmaximum density by a stamping operation at a pressure of the order of 40to 45 tons per square inch.

The contact material produced by this alternative method also exhibitslow, stable contact resistance at low contact forces for long periods intarnishing atmospheres.

It should be noted that the simultaneous sintering and oxidation ofgreen silver-ruthenium contacts results in a net shrinkage when thematerial is initially compacted at tons per square inch. At tons persquare inch a net expansion occurs.

When, during the sintering step of the methods according to theinvention, the silver recrystallized and grain growth begins, the grainsgrow until they meet a ruthenium oxide particle. The ruthenium oxideparticles impede further grain growth and remain in the grain boundariesto effectively anchor them in position. Thus the ruthenium oxide contentof the contact materials produced by the methods according to theinvention is mostly located in the grain boundaries in the silver.

The contact resistance properties of the light-duty electrical contactmaterials according to the invention in comparison with silver (Ag) andsilver-cadmium oxide (AgCdO) contact materials are indicated in thetable given below:

Contact CONTACT RESISTANCE (OHMS) Force (Grms.wt.) Silver Ag- Ag- Ag-Ag- 10% 1.3% 2.6% 3.9% CdO RuO RuO RuO z 7.0 3.32 0.36 0.165 0.150 10.32.90 0.13 0.034 0.011 0.030 13.6 2.50 0.032 0.008 0.005 0.009 17.0 2.150.014 0.009 0.009 0.003 20.3 0.28 0.011 0.006 0.006 0.003 23.6 0.200.003 0.006 0.003 27.0 0.12 0.007 0.004 0.010 0.005 30.3 0.13 0.0030.012 0.003 33.6 0.003 0.012 0.003 37.0 0.04 0.006 0.002 0.008 0.004

The contact resistance is shown as a function of contact force after 21hours exposure to a moist H 8 atmosphere i.e. an atmosphere containing700.0 mm H 5 and 17.0 mm of H 0.

It should be noted that when low tolerance contact dimensions arerequired it is important to avoid excessive shrinkage of the contactcompact during the sintering step of the methods according to theinvention. The shrinkage which occurs during the sintering step isdirectly influenced by the initial forming pressure used to press thegreen contact compacts and therefore the correct choice of the initialforming pressure is, under these circumstances, very important.

An advantage of the silver-ruthenium oxide is that it is readilysolderable with soft solder, it is readily capable of heading to form arivet, and it is capable of being brazed.

In a further alternative method according to the invention the rutheniummetal powder utilized in the methods outlined in preceding paragraphs isreplaced by ruthenium oxide powder such that the ruthenium oxide contentof the silver-ruthenium oxide mixture is in the range 0.1 to 13.0 atomicper cent. The silverruthenium oxide mixture is then compacted andsintered in an inert atmosphere in the manner outlined in precedingparagraphs. While this production method produces a silver-rutheniumoxide material that may be suitable for certain applications it is notthe preferred method because it results in a less favourable oxideparticle size distribution and an inferior dispersion of oxide particleswithin the silver matrix.

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitation in its scope.

What is claimed is:

l. A light-duty electrical contact material which consists ofa mixtureof silver and ruthenium oxide, the ruthenium oxide content being in therange 0.1 to 13.0 atomic per cent.

2. A light-duty electrical contact material as claimed in claim 1wherein the ruthenium oxide content is 1.3

atomic per cent.

2. A light-duty electrical contact material as claimed in claim 1wherein the ruthenium oxide content is 1.3 atomic per cent.